All of the publications, patents and patent applications cited within this application are herein incorporated by reference in their entirety to the same extent as if the disclosure of each individual publication, patent application or patent was specifically and individually indicated to be incorporated by reference in its entirety.
Cavity ringdown spectroscopy (CRDS) is a technique that utilizes the resonance of light within an optical cavity to allow the measurement of very small concentrations of light-absorbing analytes. The optical cavity consists of two or more highly reflective mirrors, between which the sample containing the analytes to be measured is placed. Optical resonance is achieved by excitation of the cavity using light with appropriate frequency and mode characteristics, said light usually provided by a laser source. When the light used to excite resonance in the cavity is abruptly cut off, either by a modulation device or by the inherent pulse-like nature of the light beam, the light remaining in the cavity bounces between the cavity mirrors while its intensity decreases exponentially as function of time due to the attenuation by the cavity mirrors and the absorption by analytes within the cavity. This process is known as “cavity ringdown” (reference: “Development of an Infrared Cavity Ringdown Spectroscopy Experiment and Measurements of Water Vapor Continuum Absorption”, John G. Cormier, PhD Thesis, 2002).
The rate of decay of light intensity within the cavity is usually determined by measuring the intensity of a small fraction of light exiting from one of the cavity mirrors as a pre-arranged leakage. The thousands of reflections that occur before the leakage signal intensity is too small to usefully measure create an effective beam path length several orders of magnitude longer than the separation between the mirrors. This effectively magnifies the attenuation of light by the analytes in the cavity, therefore very small concentrations of analytes can be measured.
Gas lasers have been used with photoacoustic cells for multi-component analyte detection, e.g. U.S. Pat. No. 6,363,772 by Berry where a CO overtone laser is used. Although photoacoustic techniques have been shown to be effective in the detection of trace gases (Photoacoustic Spectroscopy in Trace Gas Monitoring, Harren et. al., Encyclopedia of Analytical Chemistry, pp. 2203-2226, (J. Wiley & Sons 2000)), these techniques are not as effective in measuring absolute quantities of constituents as the cavity ring-down method. One of the reasons for this is that cavity ringdown measurements are purely ratiometric, that is they provide a means for direct measurement of light attenuation by absorbing molecules without requiring a priori knowledge of instrument parameters such as laser beam intensity, ringdown cavity length or mirror reflectivity. In contrast, photoacoustic spectroscopy is an indirect detection method where acoustic waves generated by heat fluctuations caused by the absorption of light by molecules in the sample are measured. Although potentially a very sensitive technique, photoacoustic spectroscopy suffers from several drawbacks that limit its ability to accurately measure analyte concentrations. Some of these noted in U.S. Pat. No. 5,528,040 by Lehmann:                (1) A quiet acoustic environment is required (therefore, use of an electric discharge or rapid flow of the sample leads to a substantial increase in noise);        (2) The sample is exposed to some average light flux, which can lead to photochemistry in some situations;        (3) The indirect nature of the detection makes determination of absolute absorption strengths difficult. The only practical way to calibrate the strength of the acoustic signal is to use a mixture of a gas which has some transition whose cross-section is already known along with the gas of interest. Even with such calibration, uncertainties on the order of 20% remain.        
In order to measure analyte concentrations using infrared-range cavity-enhanced laser-based devices, the prior art teaches the tuning of a laser line to the frequency of a principal absorption line of the analyte of interest, and then measure the change in some physical parameter related to the absorption of light by the analyte at said frequency. For the case of cavity ringdown measurements, the measured physical parameter is the decay time of laser light intensity in the cavity, usually determined by proxy through measurement of a small amount of light designed to leak from one of the cavity mirrors. For photoacoustic spectroscopy, the measured quantity is the change in acoustic energy due to changes in heat resulting from the absorption of light by the analyte. A reference measurement is usually also made by tuning the laser frequency away from the absorption peak. The difference between the two measurements is then used to infer the amount of gas in the cell. Such a two-frequency method is described for example in Pat. WO02/090935 by Patel, where it is used to determine the concentrations of various gaseous compounds using laser-based photoacoustic cell measurements. The disadvantages of this method are that only one analyte at a time can be measured, low gas mixture pressures are usually required, and large measurement errors can occur due to the presence of unknown analytes in the mixture.
Situations exist where it is desired to measure the composition of a complex mixture of analytes to better than part-per-billion accuracy, under ambient atmospheric temperature and pressure conditions, without prior compositional knowledge or bias. Examples of said applications include, but are not limited to, human exhaled breath measurement for the diagnosis and monitoring of medical conditions, environmental monitoring of toxins, explosives detection and industrial process monitoring. Therefore there is a need to have an apparatus that can identify and quantify multiple analytes within an unknown, complex mixture in an accurate and unbiased manner, to a very high degree of sensitivity, without the added complexity of having to pre-concentrate the analytes or modify the pressure of the mixture.